This invention generally relates to diffusion processes in silicon. The invention is particularly applicable to crystalline silicon solar cells.
Thermal diffusion is commonly used to form p-n junctions in crystalline silicon solar cells. In p-type silicon, phosphorous is diffused into the bulk silicon to form a junction. In n-type silicon, boron is diffused into the silicon to form a junction. Prior art methods of thermal diffusion include using tube furnaces to provide the heat required to drive dopant atoms into the silicon lattice structure. The dopant can be delivered to the wafer during the diffusion heating using a vapor source such as phosphorous oxychloride for a phosphorous diffusion or boron tribromide for a boron diffusion. To produce a solar cell using n-type silicon, two high-temperature diffusion processes are needed. A boron diffusion is performed on the first surface of the wafer to produce a charge separating emitter field. On the second surface of the wafer, a phosphorous diffusion is carried out to form a deep junction to act as a conductive back surface field (BSF). In conventional practice, these two processes are performed sequentially with discrete machinery which results in long cycle times, expensive capital equipment and operating costs. Also, additional process steps, such as glass removal etching and masking have to be performed during each sequential process which adds to capital, materials and operating expenses.